Estimate the planned IT load first, then model the electricity and water required by the facility that supports it. Use a project-specific operating profile, a cooling design matched to the site climate, and declared accounting boundaries. Convert IT electricity to total facility electricity with a design-appropriate Power Usage Effectiveness (PUE) assumption or simulation. Model direct site water from the selected cooling system, and report water associated with off-site electricity generation separately. Present a central case plus low and high scenarios; a national demand forecast cannot stand in for an engineering estimate for one proposed facility.
What the estimate must answer
A pre-construction utility estimate has four different outputs that should not be blended into one number:
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- IT electricity: the energy used by servers, storage, networking and other information-technology equipment.
- Total facility electricity: IT electricity plus cooling, power conversion and distribution, fans, pumps, lighting, controls and other supporting systems.
- Direct site water: water withdrawn, discharged or consumed at the facility, principally for heat rejection and related treatment.
- Source or indirect water: water used at power plants to generate the electricity purchased by the data center.
Report annual energy and water separately from peak electrical demand and peak water flow. A utility needs the peak values for service, transformers, switchgear, generators and treatment capacity; finance and environmental reporting usually need annual consumption. A nameplate maximum, an annual average and a design peak are different quantities.
Declare the boundary before doing any arithmetic
Write a one-page scope statement before selecting assumptions. It prevents two proposals from appearing comparable when one includes only the building and the other includes the power supply chain.
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| Scope item | Decision to record | Why it changes the result |
|---|---|---|
| Project stage | Concept, schematic design, detailed design or retrofit | Early estimates rely on ranges; later estimates can use equipment schedules and control sequences. |
| Location and weather | Site, elevation, design temperatures, humidity basis and weather file or station | Cooling hours, economizer availability and heat-rejection performance vary by climate. |
| IT capacity | Installed MW, usable capacity, rack density and expansion phases | Capacity is not the same as the load operating every hour. |
| Operating profile | Hourly or representative load by season, utilization, redundancy mode and growth schedule | Annual kWh and cooling load depend on when equipment runs, not just its maximum rating. |
| Electricity boundary | IT-only, building total, or building plus on-site generation losses | PUE is meaningful only when numerator and denominator use the same boundary. |
| Water boundary | Site withdrawal, site consumption, discharge, or site plus source water | Evaporation, blowdown, treatment and power-plant water are distinct flows. |
| Time basis | Annual total, monthly profile, hourly profile, and coincident peak | Infrastructure is sized to peaks while permits and budgets often use annual totals. |
State whether “water use” means withdrawal or consumption. Withdrawal is water taken from a source; consumption is the portion not returned to that source, often because it evaporates. If a permit or utility uses a particular definition, use that definition consistently and show the other quantity as a separate line.
Use the right metrics, with their equations
| Metric | Equation | Unit and interpretation |
|---|---|---|
| IT energy | Sum of IT power × time over the operating period | kWh or MWh; calculated from the load profile. |
| Facility energy | IT energy + all supporting-facility energy | kWh or MWh; the quantity purchased or generated for the site boundary. |
| PUE | Total facility electricity ÷ IT equipment electricity | Dimensionless. It is a ratio, not an annual demand forecast. |
| Site WUE | Annual on-site water consumption ÷ IT equipment electricity | Liters per kWh (L/kWh), when the site-water boundary is used. |
| Peak electrical demand | Maximum coincident facility power in the selected interval | kW or MW; include the interval, such as 15-minute or hourly peak. |
| Peak water flow | Maximum coincident make-up, treatment or discharge flow | Liters per second, gallons per minute or another stated flow unit. |
Keep PUE and WUE tied to the same IT denominator when comparing alternatives. A design can have a lower PUE but higher site water use, or the reverse. Report both ratios and the underlying absolute electricity and water quantities.
Step 1: Build an IT electricity load profile
Inventory the proposed equipment
List server, storage, networking and special-purpose equipment by room or electrical bus. For each class, record quantity, expected operating power, standby power, power-supply efficiency and planned replacement or expansion. A vendor nameplate is a useful upper bound, but it is not an annual consumption estimate. Confirm whether the rating is input power, output power, thermal design power or a maximum under a particular workload.
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Represent utilization over time
Use an hourly profile when the project has variable workloads, staged occupancy, seasonal operations or demand-response plans. If hourly data are unavailable, create representative weekday, weekend and seasonal blocks and document the assumed utilization in each block. Calculate both an average and a coincident peak. Do not multiply installed MW by 8,760 hours unless the equipment is actually expected to operate at that level continuously.
Include growth and redundancy
Separate commissioned load from ultimate build-out. Show the year or phase in which each hall, rack group and power train is energized. Redundant equipment may draw standby power even when it is not carrying IT work; include that draw if it is inside the electrical boundary. If the design reserves capacity for future racks, show reserved capacity separately from energized IT demand.
Convert the profile to energy and peak demand
For each interval, multiply IT power by the interval duration and sum the results for annual IT kWh. Retain the highest coincident IT value for electrical sizing. Document whether peak means a normal operating peak, a commissioning peak, a contingency condition or a short-duration inrush event; each has different equipment implications.
Step 2: Add facility electricity with PUE or an infrastructure model
Use PUE as a calculated result when possible
PUE equals total data-center electricity divided by IT equipment electricity. For a concept estimate, apply a range that is explicitly tied to the proposed cooling, power and control design. For later design stages, model the supporting systems directly and calculate the resulting PUE by hour or month. Never present a generic industry benchmark as a guaranteed value for a future site.
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Model every material supporting load
The infrastructure representation used in the 2024 Lawrence Berkeley National Laboratory (LBNL) analysis includes cooling and equipment such as uninterruptible power supplies, power transformation and distribution, fans and pumps. A project model should also identify chillers, heat-rejection devices, humidification or dehumidification, lighting, controls, battery charging, fuel-system auxiliaries and any water-treatment equipment that draws electricity.
Account for part-load behavior
Cooling and power equipment rarely operate at their full-load efficiency for every hour. Use manufacturer curves or a validated engineering model where available. Include staging, variable-speed drives, setpoint resets, economizer operation and the effect of redundancy. If those data are not yet available, show a wider scenario range rather than implying precision.
Calculate annual facility energy
For each interval, calculate IT power and supporting-facility power, then sum both. The annual facility total divided by annual IT energy gives an annual PUE. Also calculate monthly PUE or a load-weighted hourly result when seasonal performance matters; an annual ratio can conceal a short period of very high overhead.
Step 3: Select and model the cooling architecture
Identify the heat-removal path
Describe where heat goes at each operating mode: air-side economizer, chilled-water loop, direct expansion, evaporative heat rejection, dry cooler, liquid cooling or a hybrid sequence. The cooling choice affects both electricity and water, and no single architecture is most efficient in every location.
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Use local weather and operating constraints
Thermodynamic simulations should use weather data appropriate to the site. The 2024 LBNL report evaluates multiple cooling-system categories with weather-station data to derive PUE and site-WUE estimates. That approach is useful for comparing scenarios, but its results do not determine the values for an unspecified project. Select design temperatures, humidity limits, air-quality requirements and water temperature assumptions that match the proposed facility.
Model economizer hours honestly
Air-side economizing can reduce mechanical cooling when outdoor conditions are suitable. Its potential depends on climate, acceptable air quality, humidity limits, filtration, control setpoints and the IT equipment’s allowable inlet conditions. Count only the hours in which the controls can actually use outside air; do not apply a fixed annual percentage without a weather and controls calculation.
Separate sensible and latent loads
IT equipment is primarily a sensible heat load, while outside-air systems can introduce latent loads. Humidification and dehumidification may materially change both electricity and water. Record the indoor temperature and humidity range, allowable excursion, filtration pressure drop and any special process loads before finalizing the cooling estimate.
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Step 4: Estimate direct site water
Cooling-tower water
For evaporative heat rejection, calculate make-up water as evaporation plus blowdown and other treatment losses, less any approved recovery or reuse. Evaporation rises with heat rejected and ambient conditions. Blowdown is drained to control dissolved minerals; its rate depends on cycles of concentration, water chemistry and treatment strategy. U.S. Department of Energy Federal Energy Management Program (FEMP) guidance explains these mechanisms and emphasizes that water demand depends on heat load and the efficiency of heat-removal steps.
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Dry and hybrid heat rejection
Dry coolers can reduce on-site water consumption but may require more fan or chiller electricity during hot conditions. Hybrid systems can switch between dry and evaporative modes, so the estimate must include the control sequence and the weather conditions that trigger each mode. Report the electricity-water trade-off instead of labeling one mode universally superior.
Water treatment and reuse
Identify the source-water quality, treatment train, cycles of concentration, discharge limits and any use of reclaimed or harvested water. A reuse system may reduce potable-water withdrawal while leaving total site consumption largely unchanged if the reused water would otherwise have been discharged. Show source, make-up, blowdown, discharge and consumption as separate lines in the water balance.
Liquid cooling and ancillary loads
If direct-to-chip or immersion systems are proposed, include the coolant loop, heat exchangers, dry or evaporative heat rejection and maintenance losses. Do not assume that moving heat from air to liquid eliminates water demand; it changes where the heat is rejected. Include humidification, domestic use, laboratory or testing loads and fire-system testing only when they are part of the declared facility boundary.
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Electricity generation can have a water footprint that is separate from water used at the data-center site. The 2024 LBNL work distinguishes site WUE from source water and models location-specific water use from both on-site cooling and electricity generation under different cooling and power-supply scenarios.
To estimate this component, document the electricity supply mix, the geographic and temporal accounting method, and the source-water factors used. Keep the result in a separate column from direct site water. A site with nearly zero operational cooling water can still be associated with source water through its electricity supply, while a water-intensive cooling design can have high direct use even when the grid’s source-water factor is low.
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Step 6: Produce a range instead of an unsupported single point
Create at least three cases with the same reporting boundaries:
| Case | Typical purpose | Assumptions to vary |
|---|---|---|
| Low demand | Efficient operation or slower occupancy | Lower IT utilization, favorable weather, efficient controls and lower water intensity consistent with the design. |
| Central design | Budgeting and utility discussions | Most likely occupancy, weather basis and equipment performance supported by current design information. |
| High demand | Service, permit and resilience planning | Higher coincident IT load, less favorable weather, degraded part-load performance or conservative water-treatment assumptions. |
Do not change several assumptions silently between cases. For every case, list site and weather file, IT profile, cooling architecture, PUE or modeled infrastructure, direct site water, indirect-water boundary, annual period and peak interval. A range is useful only when the reader can see what caused the spread.
Worked illustrative calculation
The following arithmetic demonstrates the method; it is not a forecast for a particular facility. Assume a hypothetical project with 10 MW of installed IT capacity, a 60% average utilization over 8,760 hours, an illustrative annual PUE of 1.30 and a site-WUE assumption of 0.50 L/kWh.
| Calculation | Result |
|---|---|
| Annual IT energy: 10 MW × 0.60 × 8,760 h | 52,560 MWh per year |
| Annual facility energy: 52,560 MWh × 1.30 | 68,328 MWh per year |
| Direct site water: 52,560,000 kWh × 0.50 L/kWh | 26,280,000 L per year (26,280 m³) |
| Illustrative facility peak from a 10 MW IT peak × 1.30 | 13 MW, only if the stated PUE is valid at that peak |
The example deliberately exposes its assumptions. In an actual design, PUE may vary by hour, the 60% average may not describe the peak month, and WUE should come from the selected cooling system and local weather model. If source water is included, calculate it as a separate addition using the documented electricity-supply factors.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Compare sites and cooling designs on the same axes
Use a comparison table in which every option has identical IT capacity, utilization profile, weather basis, accounting period and water boundary.
| Comparison axis | Question to answer |
|---|---|
| Electrical capacity | What are annual facility MWh, normal peak MW and contingency peak MW? |
| Facility overhead | What PUE results from the modeled cooling, power and controls at annual and peak conditions? |
| Direct water | What are annual withdrawal, consumption, discharge and peak make-up flow? |
| Site WUE | What is annual site water consumption per IT kWh? |
| Source water | What electricity-generation water is added under the selected supply scenario? |
| Climate response | How many hours support economizing, and how does performance change on design-hot days? |
| Resilience | What happens to electricity and water when a redundant chiller, tower cell, pump or power train is unavailable? |
| Expandability | How do the values change as later halls or higher-density racks are commissioned? |
Normalize the alternatives before ranking them. Comparing one design at 40% utilization with another at 70%, or one site with source water included and another with site water only, produces a misleading result.
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Hot- and cold-aisle separation
FEMP identifies hot- and cold-aisle separation as a way to contain heat, support higher chilled-water temperatures and reduce unnecessary airflow. The same guidance attributes a potential 20% reduction in chiller energy to these practices and notes that lower heat rejection can also reduce cooling-tower water needs. Treat that figure as a cited potential outcome, not a guaranteed saving; the result depends on containment quality, rack layout, setpoints, controls and the existing baseline.
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Higher chilled-water temperatures and variable flow
Test reset schedules, variable-speed pumps and fans, and the effect of higher supply-water temperatures in the thermodynamic model. Verify that rack inlet temperatures, humidity and equipment warranties remain within limits. Savings in one subsystem can increase another subsystem’s load, so evaluate the complete facility rather than a chiller in isolation.
Economizer controls
Evaluate air-side or water-side economizers against the site weather file and air-quality requirements. Include filter pressure drop, fan energy, humidity control, smoke events and the switchover logic. The result may be lower mechanical cooling energy and water use in suitable hours, but the model must show when those hours occur.
Water chemistry and cycles of concentration
Increasing cycles of concentration can reduce blowdown, but only within the limits of source-water chemistry, treatment capability, corrosion control and discharge permits. Have the water-treatment designer verify the assumption before using it in a water budget.
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LBNL’s 2025 update projects that data centers could account for 11.8% of total U.S. electricity use by 2030, with scenarios ranging from 9.5% to 15.3%. Those are national scenarios, not a multiplier for a planned building and not evidence of a particular site’s annual demand. They are useful for explaining why grid capacity and regional planning matter, but project decisions still require the IT, cooling, weather and operating inputs described above.
A DOE explanatory article also reports that an NREL data center dedicated 6% of its energy consumption to equipment cooling and says a typical data center requires 70% for that task. Because the article does not establish a common project-specific definition and boundary for those two figures, treat them as illustrative remarks rather than forecasting inputs. Do not use either percentage to size an unspecified facility.
Common estimation failures and how to correct them
- Using installed IT MW as annual energy. Build an operating profile and integrate it over time.
- Applying one generic PUE. Tie PUE to the proposed power, cooling, controls, redundancy and climate, or model those systems directly.
- Reporting only tower make-up. Show evaporation, blowdown, treatment losses, discharge and consumption so the water boundary is visible.
- Mixing site and source water. Keep on-site water and electricity-generation water in separate columns and state whether both are included.
- Ignoring part-load operation. Use equipment performance curves, staging logic and seasonal weather rather than design-day values for every hour.
- Hiding phase growth. Show each commissioning phase and the ultimate build-out as separate cases.
- Comparing unequal scenarios. Hold IT load, utilization, weather basis, period and boundaries constant when comparing sites or cooling systems.
- Giving a false-precision result. Publish a central estimate plus low and high cases, with the assumptions that drive the range.
- Forgetting peak conditions. Provide normal, design and contingency peaks with their interval and operating condition.
- Assuming an efficiency measure is automatic. Verify controls, maintenance, air quality, humidity, water chemistry and operator practice before crediting savings.
Information to secure before the design is frozen
The estimate is ready for utility and permitting discussions when the project team can provide a traceable input set:
- Room-by-room IT equipment schedule, power basis and rack-density plan.
- Hourly or representative seasonal utilization profile, including staged occupancy and growth.
- Electrical one-line diagram with conversion losses, UPS modes, distribution losses and on-site generation assumptions.
- Cooling schematic, equipment capacities, redundancy mode, control sequences and part-load performance.
- Site weather file or station, design temperatures, humidity limits, air-quality requirements and economizer rules.
- Water balance identifying source, treatment, make-up, evaporation, blowdown, discharge, reuse and consumption.
- Electricity supply mix and accounting method if source water is reported.
- Annual totals, monthly or hourly profiles, normal peaks and contingency peaks for every scenario.
- Version, date and owner for each assumption so later design changes can be reconciled.
Have the mechanical, electrical, controls and water-treatment engineers review the same model. A change to rack density can alter IT power, which alters heat rejection, which alters both facility electricity and tower water. Keeping the assumptions linked avoids a water estimate that no longer matches the electrical design.
Bottom-line method
A defensible pre-construction estimate is a transparent chain: define the site and boundaries; model IT load and utilization; add facility overhead through PUE or a system simulation; calculate cooling electricity and direct water with local weather and operating controls; add source water only when required; and publish comparable low, central and high scenarios with annual and peak values. Without the planned IT profile, location, cooling architecture, power-supply assumptions and accounting boundary, no single annual electricity or water total is credible.
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